1. Field of the Invention
The present invention relates to an optical fiber cable, and more particularly to an optical fiber cable installed in a duct using an air-blown installation method.
2. Description of the Related Art
In general, in order to install an optical fiber cable in a duct, methods for pushing or pulling the optical fiber cable into/from a predetermined end of the duct have been used. However, such installing methods may apply excessive stress to the optical fiber cable causing damage. Particularly, in the case of installing the optical fiber cable in a duct, an optical fiber accommodated in an optical fiber cable may be affected from such stress, thereby forming a minute curvature on a surface of the optical fiber cable while applying residual stress to the surface of the optical fiber cable. Accordingly, the optical fiber cable installed through such installation methods must include various tensile members in order to improve tension-resistant force.
As a replacement for such installation methods, an air-blown installation, which installs the optical fiber cable by blowing air into the duct, has been used. The air-blown installation method has advantages in that stress is rarely applied to an internal optical signal transmitting medium. This allows for the number of tensile members of the optical fiber cable to be reduced. The optical fiber cable installed through the air-blown installation method, however, must have a smaller diameter and a lighter weight than those of the optical fiber cable installed through the other installation method described above. In addition, the optical fiber cable must be easily moved when air is applied thereto.
An optical fiber cable disclosed in U.S. Pat. No. 5,389,442, which is issued to Candido J. Arroyo and entitled “Water blocking strength members”, includes a plurality of tubes and a plurality of auxiliary waterproof members interposed between the tubes. The above tubes and auxiliary waterproof members are surrounded by two layers of rigid yarn. These two layers are surrounded by a sheath. The optical fiber cable maintains a circular sectional shape due to internal parts thereof even if the thickness of the sheath becomes thin.
Conventional optical fiber cables capable of being installed by an air-blown installation method must have an outer diameter that is less than 80 percent of an inner diameter of a small duct. This is therefore a design limitation for the optical fiber cables. However, if the optical fiber cable using with the air-blown installation method is designed identically to the optical fiber cable for the tension installation method, it is difficult to obtain a suitable outer diameter of the optical fiber cable for the air-blown installation. Also, an improper design may be created in view of maintain tension and waterproof characteristics.
Accordingly, an improved optical fiber cable for the air-blown installation is needed in the art.